Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
Ceramics made from organometallic polymers such as organosilicon polymers have the potential to overcome these problems. To this end, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See, for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
The major and most critical application for ceramics based on polymer processing is high strength, high modulus, reinforcing fibers. Such fibers are spun from organosilicon preceramic polymers and are subsequently converted to ceramic materials, in particular, silicon carbide/silicon nitride bearing fibers by a two-step process of curing to render the preceramic polymer fiber insoluble followed by a routine pyrolyzation schedule comprising heating the fiber up to about 1,200.degree. C. whereupon the fiber is converted to the ceramic form.
Other metallic polymers have recently been suggested as ceramic precursers besides organosilicon polymers. Thus, U.S. Pat. No. 4,581,461 forms boron nitride by pyrolyzing B-triamino-N-tris (trialkylsilyl)borazines. U.S. Pat. No. 4,097,294 suggests that a boron carbide ceramic is obtainable from a carborane carbon polymer.
The formation of aluminum nitride fibers is disclosed in commonly assigned, U.S. Pat. No. 4,687,657. Aluminum nitride ceramics are formed by thermal conversion of poly-N-alkyliminoalanes. Ceramics comprising silicon carbide and aluminum nitride solid solutions are also disclosed. These ceramic alloys are formed by thermal conversion of a mixture of an organosilicon preceramic polymer and the above-mentioned aluminum-containing polymer. Moreover, many recent patents describe specific silicon-containing preceramic polymers which are formed into silicon carbide and/or nitride upon thermal treatment.
Alternatively, ceramic fibers such as metal carbide fibers have been formed by incorporating inorganic metallic compounds into a carbon fiber product, the precarbonaceous polymer forming solution, the polymer spinning solution or the polymer fiber subsequent to spinning, and converting the metallic compounds in situ to metal carbides upon thermal conversion. In these methods, the precarbonaceous polymer acts as the source of carbon.
Important ceramics formed by such method are boron carbide and boron carbide-containing carbon fibers. The addition of boron carbide to carbon fiber is known to increase fiber strength and, more particularly, to substantially increase the thermo-oxidative stability of carbon fibers such that the boron carbide-containing carbon fibers can withstand higher temperature environments than carbon fibers. Methods of incorporating boron into carbon fibers to form boron carbide fibers have typically involved treating the carbon fibers with gaseous boron halides or impregnation with soluble borane salts or boric oxides including boric acid, metallic borates and organic borates, e.g. alkyl and aryl borates. Upon being treated with the boron compounds, the fibers are heated to initiate reaction of boron with the carbon fibers to yield boron carbide.
In commonly assigned, copending application U.S. Ser. No. 933,413, filed Nov. 21, 1986, now abandoned and continuation-in-part application U.S. Ser. No. 082,761, filed Aug. 7, 1987, boron-containing fibers are provided by forming a blend of a boron-containing polymer and a precarbonaceous polymer, shaping the blend into a fiber such as by spinning and pyrolyzing to form a boron ceramic fiber. Preferably, the boron-containing polymers are prepared by the condensation of boranes with Lewis bases. Such polymers are well known and prepared by condensing a borane such as diborane, pentaborane or decaborane with Lewis bases such as amines, amides, isocyanates, nitriles and phosphines. The borane-Lewis base condensation polymers are known and described, for example, in Polymer Letters, Vol. 2, pp. 987-989 (1964); Chemical Society (London) Spec. Publ. No. 15 (1961), "Types of Polymer Combination among the Non-metallic Elements", Anton B. Burg, pp. 17-31; U.S. Pat. Nos. 2,925,440; 3,025,326; 3,035,949; 3,071,552; and British Pat. No. 912,530. Other borane-containing polymers suggested include those disclosed in U.S. Pat. No. 3,441,389 wherein borane polymers are prepared by heating a compound of the formula (RAH.sub.3).sub.2 B.sub.10 H.sub.10 or (RAH.sub.3).sub.2 B.sub.12 H.sub.12 at a temperature of 200.degree.-400.degree. C. for several hours. Moreover, borazines such as disclosed in U.S. Pat No. 4,581,468 and carborane polymers such as suggested in U.S. Pat. No. 4,097,294 are also considered useful.
The use of organometallic polymers as precursors for ceramic materials is advantageous in the formation of ceramic fibers. It is considerably easier to spin the polymeric materials than precursors composed of inorganic metallic particles dispersed in a spinnable organic matrix. It would, therefore, be desirable to find new organometallic polymers and methods of making same which can be used as ceramic precursors. The present invention is concerned with preparing organoboron polymers which can serve as precursors for boron ceramics such as boron carbide and boron nitride and ultimately to the formation of fibers containing these boron-containing ceramic materials.
One difficulty in preparing boron-containing ceramics from organic precursers is the inability to incorporate sufficient boron into the organic polymer and react with the carbon components to form boron carbide, B.sub.4 C. Methods of incorporating boron-containing salts or boron-containing inorganic powders and the like into precarbonaceous polymer solutions, solids, or the formed carbon articles have proved unsuccessful in providing sufficient amounts of boron to yield improved boron carbide-containing ceramic materials. There is, therefore, a continuing need to find additional preceramic organoboron polymeric materials which yield ceramics containing increased levels of boron.
Although, as described above, decarborane-containing polymers such as those produced by the reaction of decaborane with a Lewis base such as an amine, amide, nitrile, etc. were prepared in the early 1960's and thought to be useful as high temperature stable polymers or even as high energy fuels, their use as a boron-containing ceramic precursor was not recognized. Accordingly, it is a primary object of the present invention to utilize the polymers produced by the condensation of a borane with a Lewis base as a boron-containing ceramic precursor.